A two-stroke engine can be built simple and light and with a high power in relation to its low weight. This has resulted in that it is frequently used for portable working tools, particularly since a crankcase scavenged engine can be equipped with a simple and all-position lubrication system. A well-known problem is however that air/fuel mixture will be lost through the exhaust port during the scavenging process of the engine. This results in increased fuel consumption as well as increased exhaust emissions. A way to reduce this problem is described in U.S. Pat. No. 4,253,433. The engine is equipped with at least one additional scavenging duct. This debouches beyond the other scavenging duct as seen from the exhaust port. The other scavenging ducts are scavenging clean air from the crankcase and this air is intended for use as a buffer in order to prevent the scavenging gases released from the additional scavenging duct, which contain air/fuel mixture, from reaching the exhaust port. This can accordingly be described as a stratified scavenging that is stratified in space. The engine has two completely separate air inlets and the one connects to the crankcase for feeding this with fresh air. The other air inlet is provided with a carburetor and debouches into the additional scavenging duct, so that air/fuel-mixture will be sucked down into this scavenging duct when the piston is moving upwards in its intake stroke. When the piston later on is moving downwards in its working stroke the air/fuel-mixture will be scavenged through the port of the additional scavenging duct. Each air inlet is provided with a valve called C and D respectively, which is located where the inlet connects to the additional scavenging duct and to the crankcase respectively. These valves are usually of check valve type, so called Reed-valves. However, the valves C and D could also be valves driven and controlled by the piston's movement or by the crankshaft's rotation. This will however be much more complicated than using the automatic check valves. Both air inlets must be provided with at least one throttle valve each and the movement of these throttle valves must be synchronized, e.g. by using one or more external link rods. Obviously this would be complicated, costly and rather sensitive. Furthermore, the additional scavenging duct is arranged as an integrated part of the cylinder and the crankcase. This means that die-casting of the cylinder would be either impossible or considerably more complicated to carry out, thus resulting in an expensive cylinder.
DE 2650834 shows an engine equipped with additional scavenging ducts intended for a rich air/fuel mixture. These scavenging ducts are located beyond the scavenging ducts for scavenging of fresh air, so that also here a stratified scavenging in space can be achieved. All scavenging ducts are closed, so that die-casting would be impossible or considerably more complicated to carry out. Also this engine has two completely separate air inlets, each of them equipped with at least one throttle valve, which thus must be synchronized. Consequently, the engine has a duct intended for a rich mixture released from an auxiliary carburetor and this duct connects to the additional scavenging ducts by way of check valves or by control of the piston in one embodiment. This means a simplification compared to U.S. Pat. No. 4,253,433, at least for those variants where the valves C and D are driven and controlled by the piston motion or by the crank motion.
Common to both these solutions is that the scavenging will create a stratification in space so that fresh air will be scavenged closest to the exhaust port and hopefully prevent the air/fuel-mixture from reaching the exhaust port. Experience shows however that this cannot be fully prevented meaning that the air/fuel-mixture will manage to reach the exhaust port and thereby be lost through this. Obviously, this will occur to a much smaller extent than in a conventional engine, but of course this is still undesirable.